Core update
Update de.ts

revised all of german translation.
fixed some original translation strings
adjusted translation messages

as discusses here: https://github.com/tux3/qTox/pull/848#discussion_r20851990
Merge branch 'pr856'
Merge branch 'pr858'
Merge branch 'pr798'
Group chat button at the bottom of contact list resized to 16x16 like other buttons
Merge branch 'pr861'
Fix some nullptr dereferences in Audio

Fixes #859
Italian translation: update